The excellent high temperature mechanical properties of polyimide resins have led to their increased use in high performance applications, particularly in the field of fiber reinforced resin matrix composite materials.
Both thermoplastic and thermosetting polyimide resins are available commercially. The thermosetting resins are the more widely used, despite significant processing difficulties associated with thermosetting resins. A method of processing thermosetting polyimide resins known as the Polymerization of Monomeric Reactants (PMR) employs an easily evaporated solvent, e.g. methanol, to dissolve monomeric polyimide precursors and form a low viscosity solution for fiber impregnation. Typically, reinforcing fibers are impregnated with a solution of monomeric reactants, the solvent is evaporated and the reactants are cured by heating the impregnated fibers under pressure in an autoclave or in a compression mold to form a fiber reinforced polyimide matrix article. In both processes, volatile components, i.e. solvents, impurities and reaction products may become trapped within the resin during cure and cause internal voids and surface blisters.
What is needed in the art is a method for making a crosslinked polyimide that is substantially free of internal voids and surface blisters.